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What is the true formula for parasite drag?
I found this site with formulas to calculate for the parasite drag. But I wondered if these formulas are to calculate parasite drag or the parasite drag coefficient? So do you have to multiply the calculated value with the square of the velocity, as in the formula for the general drag force?
I also wonder if there is a difference between the wetted area of an airfoil and the normal planform area?

I mean this formula: f = kc_fS_wet

The philosophy employed on the Stanford site is to convert the product of drag coefficient and corresponding surface area to a "drag area", which is technically the area of a body with a drag coefficient of 1. See page 1-8 in Sighard Hoerner's book "Fluid Dynamic Drag" for the definition and tons of helpful real-world data on drag.
To convert this into a drag force, it still needs to be multiplied with the dynamic pressure, which is the product of the square of the velocity, the air density and the factor 0.5. No further inclusion of the surface area is needed, as this is already factored into the drag area values of each drag component.
Using the drag coefficient lets you compare the relative dragginess of drag components (friction drag, pressure drag, ...). If you want to compare the dragginess of aircraft components (wing, fuselage, tail surfaces, ...), using their drag area adds a factor for their size, making it easier to compare the relative influence of the drag of different aircraft components on total drag.
To answer your second question: Planform area is the wing area you see in the top view of an aircraft, and the wetted area is the actual area which is exposed to airflow. If you start with the planform area, you need first to subtract the part of the wing which is inside the fuselage. Next, you double the result to account for both sides, top and bottom, and then you divide this sum by the cosine of the dihedral angle and the cosine of the incidence angle to arrive at your wetted area.

So if i get it right, the draf coefficient already has a factor of relative dragginess in it, like S_object/S_total. So the drag area is proportional to S_object^2/S_total?

@SimonRavelingien: The drag coefficient itself is dimensionless, so it is independent of scale, area or speed. The drag area is no dimensionless coefficient any more, but depends on the size of the corresponding part $S_{object}$. By multiplying it with dynamic pressure, you get the drag force of the part. Add all parts up, and you have the total drag. Only when you want to get one drag coefficient for the whole airplane, you need to scale the contributions of single parts with their relative area $\frac{S_{object}}{S_{total}}$. A factor like $\frac{S^2_{object}}{S_{total}}$ is never needed.


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How many plane crashes are prevented by pilots?
Statistics about plane accidents consistently show pilot (or other human) error as the leading cause of crashes. With current advances in technology, it is therefore tempting to strive towards replacement of pilots with automated systems. However, I have never seen a tally of how many plane crashes are actually prevented by pilot skill, that would otherwise have resulted in loss of life. Are there any statistics available on this subject?

That's hard to say as you can count anything as crash prevention. And it is hard to define an "almost" crash. Is a TCAS warning already considered an almost crash? That's probably why you don't find many statistics. But you'll find plenty of incidents where a fully automated system would probably have crashed, yet the pilot managed to land safely (Hudson River, LOT twisted nose wheel landing and so on)

Perhaps your question should ask about incidents where the human pilot correctly took manual control because the auto-pilot wouldn't have been able to handle things properly.

As an accident is (or is usually) a chain of events, preventing an accident means preventing at least one of those events to occurs, even if the public will see only the last event as decisive. How many events have been prevented silently? This is not possible to answer this. Prevention is a permanent role of the crew. We should thanks the crews at each flight, not only after an heroic action to prevent the last event of the chain.

To me it seems, if you want to make this question less opinion based it will end up to be something like: "What are some incidents where a pilot could not have been replaced by automatic systems" which would then lead to this question. I have not found a list of published occurrences nor can I find a number, yet I'd say in about 10%-20% of all flights made, an automatic system may not have been able to recover or interrupt the chain of events mentioned by @mins

All of them, except those that aren't.

Any manually-flown landing would meet your requirements. If the pilot does nothing on final approach, the plane will almost certainly come to a fiery end.
Landings that were very, very close to a crash aren't reported all that often - if you made some monumental screwup and managed to salvage it, would you write it up for the FAA? Not many people report almost-car-accidents or red light violations either.

Yes, I suppose any dull, nominal flight could be considered due to pilot skill. But those are also typically flights that could be performed just as well by a fully autonomous plane, even with today's technology. I'm more looking to compare cases where the presence of a human caused an accident in the absence of any other off-nominal circumstances versus cases where the presence of a human prevented an accident from happening where something was off-nominal.
So that would, for example, include cases where a pilot prevented a crash when a mechanical failure would have resulted in a crash

Unlike driving a car, the pilots are legally required to report all near-misses to the FAA. Airline safety management systems rely on honest reporting without fear of repurcussions (unless the mistake was really negligent). But with two or more pilots plus ATC plus airport fire services watching, you can't exactly hide big mistakes...

Near-misses involve another aircraft, and at airliner speeds cannot be avoided without technology so surviving one here doesn't count. Regarding witnesses, if you stalled your 182 on approach to an unmanned country strip, pulled it out just before impact and tapped the approach light with a wingtip before thumping it down on one wheel, would you write it up or simply go home for fresh underwear and never speak of it again?

This was close to my first thought on the matter:  Nearly every single flight is kept from crashing by the presence of a pilot, and it isn't limited just to the landings either. Poor pilot technique can just as easily cause a crash during the takeoff roll, during the climb, cruise, or descent. Not to make light of the question, but that is what the pilot typically does: fly it from point a to point b without crashing. That's the job.

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Is all jet fuel the same?
I'm a Chemistry student preparing an in-class lab demonstration that involves the combustion properties of jet fuel used in typical US based airliners.
Is all jet fuel the same, or do I need to consider specific blends when designing my experiment? Is there a standard chemical formula for jet fuel?
And for bonus points, are there any critical handling considerations?

about that last point; contact the fire department in advance, they will probably have a foam truck standing right next to it in case things go wrong.

Your reference to the Twin Towers is conspiratorial at best.  The collapse mechanism is well understood and replicatable.  It does not need steel to melt, only to soften and lose strength which is easy to do with a big enough fire.

@Simon Oh I know man it's just to get credit for this class. I don't actually care about the conspiracy this is just clever ya feel?

Is this question not on topic? It has two down votes and I thought it was right on point?

it is badly off-topic: the question has nothing to do with aviation, except that the fuel is used in aviation.

@Federico Oh see I thought this stack was basically for all aspects of aviation. That would be totally my mistake sorry about that guys. Should I remove this question? I feel like it could be useful to future people with this question but I got my answer and I'll remove if you guys think I should...

So is this flagged because of the section in the help center about: "Resource location
Where to buy a specific part, "Which headset is the best?", etc." yeah? So should I remove my question or not? It is what I was intending to ask and it very well could be of use to future questioners but I'll remove it if that's necessary...

please read the on-topic page. your question is not within a single one of the on-topic points.

There are different types of Jet fuel. See Jet-A, Jet-A1, Jet B, and the JP fuels used for military operations. Regarding the burning properties, there is no specific 'burn temperature' for jet fuel (or any fuel, for that matter.) It will burn at any temperature above the one at which sufficient activation energy is supplied. If you burn enough of a fuel, you can reach almost any temperature you want. The "jet fuel doesn't burn hot enough to melt steel" stuff was nonsense made up by conspiracy theorists who don't understand either physics or chemistry.

@reirab: There are different types of Jet fuel, but civilian aircraft always use Jet-A in USA and Jet-A1 in most other places and the difference is very small anyway. +1 for the rest; it can burn at almost any temperature given the right conditions.

@reirab -- well, fuels are limited by their adiabatic flame temperature; however, the "jet fuel doesn't burn hot enough to cook/spaghettify structural steel" balderdash is just that -- propane has a lower adiabatic flame temperature than kerosene/jet fuel, and a big enough propane torch will make light work of unprotected structural steel!

@UnrecognizedFallingObject True, though keep in mind that smashing a giant fuel tank into a concrete/steel structure isn't a constant pressure scenario like what you have with a torch.

@reirab -- indeed; I'm just saying that I seriously doubt that you couldn't get a fire hot enough to do cook steel with jet fuel.

The fuel used by civilian airlines, fueled in the USA, is Jet A. To acquire some, go to a nearby airport which can handle operations by Turboprop or Turbofan aircraft. Some of the smaller airports, only supporting piston aircraft operations, may not be able to help you, but anything which can handle a Lear Jet should be able to supply you with examples of it.
There's no specific chemical formula for this. It is a mixture of a variety of compounds. And no, you can't isolate just one particular compound and use it as a representative sample. There are reasons for the various additives, and all of them will have SOME effect on the burn characteristics.
Try not to breathe the fumes, before or after combustion. Do not operate any kind of radio transmitter (including a cellphone) within 50 feet of the vapors of it (when the fuel is exposed to air, before or during combustion). Make sure that whatever vessel you're going to burn it in is metal (plastic doesn't respond well to burning petrochemicals :-) and make sure there is good electrical conductivity between the vessel and the ground. Have a large, CO2- or Halon-based (NOT water or dry chemical) fire extinguisher nearby.
When I was refueling F-16s in the Air Force:

no radio transmitters (walkie talkies, at the time) were allowed with 50 feet of refueling operation
the aircraft and the refueling vehicle BOTH had to be grounded and connected to each other BEFORE connecting the refueling line
no refueling ops were permitted if there was lightning within 5 miles
no refuelings ops were permitted without a 50-pound halon-based fire extinguisher and a dedicated person (fire guard) to man it

This should give you some idea of proper safety around this stuff.

Thank you so much for your response. This is quickly becoming too much for my setting. This stack has saved me so much time by showing me this would be more difficult than I was expecting. Thanks so much man.

@wposeyjr - you thought burning jet fuel would be a "beaker, bunsen burner, job done" kind of experiment? oooohkay!

@Jamiec yeah. I thought it'd be a bit more difficult because it has such a high burning temperature but I wasn't aware that it would take quite this much haha. My bad honestly. Just unaware!

Also note that if you opt for the halon-based fire extinguishers to bring along a PBE (portable breathing equipment) so you don't inhale the halon.

Well, when manipulating a quart of it in laboratory, you don't have to be so careful about static electricity (no radios, grounding) like you have to be when loading couple of tons in an aircraft outside. You still should have a fire extinguisher with good rating for extinguishing burning liquids (type B), but you need one when handling gasoline (which is more dangerous than Jet-A) anyway.

Thanks, Meower68. Could you elaborate on why ground refueling procedures seem more stringent than in-air refueling procedures? Thanks.

The main problem you're dealing with is fuel vapors.  When you're pumping fuel into the plane, there's a vent under the wing where vapors are pushed out from the fuel tanks as liquid fuel is pushed in.  And since the vapors are heavier than normal air, they can "pool" in the vicinity of the jet during fueling.  Any static discharge, in the presence of such vapors, can result in a fire and/or explosion.  At 10k+ feet and 100+ knots, there's not much probability of pooling vapors.  Also, the IFR receptacle is on TOP of the jet while the ground receptacle is underneath.

If you have any intention of doing this, and I DON'T ADVISE YOU LIGHT A POOL OF JET FUEL ON FIRE... EVER...
But if you insist take a look at this Material Safety Data Sheet which outlines all the safety and emergency aspects of Jet-A as well as other important info on it (provided by Hess).
One extremely important thing you must note is that the flash point (point where the fuel becomes vaporous and can ignite) is only 100°F this is not very high and a temperature that can be achieved in common settings. So make sure to transport it safely.
The Airport Facility Directory will tell you what fuel every airport has and will list it as either Jet-A1 or Jet-A. You may also see 100LL or 110LL; this is not what you want, this is for piston engines.
TALK TO A FIREMAN BEFORE DOING THIS!! I am still advising that you avoid this in the first place but if you insist please consult with a fireman (potentially from an airport) on the proper things you will need to put out a fire and how and where the best place to do this is. You should assume you will need to be outdoors.

Flash point is 100°F. Compared to minus 45°F for gasoline…

@Jan, you are correct I was simply mentioning that it could become vaporous on a hot day or in a hot car. All fuels should be transported in approved containers with care and should avoid direct sunlight.

@Dave It can also become vaporous when exposed to burning Jet-A... That tends to get things above 100 F pretty quickly.

@reirab But with a fire already burning wouldn't it tend to combust immediately rather than forming a mixture with a large volume of air? Plus, there's a difference between transportation (in his vehicle) vs the setting in which it is being deliberately ignited (hopefully not around anything else flammable for a good distance around).

@Random832 Immediately upon vaporizing in the presence of a flame, yes, provided that sufficient oxidizer is available. So, it won't fill the room, then explode all at once, if that's what you're asking.

As the other answers have said, you really really don't want to try this experiment yourself, and especially not indoors, but in fact jet fuel is surprisingly difficult to ignite, unless you atomize it or vaporize it first. The safety hazard comes from the fact that it is even more difficult to extinguish a jet fuel fire than to light it.
The standard safety demonstration by our works fire department includes dropping a lighted cigarette into a bucketful of fuel, which has no effect whatever. The start of this video is a more dangerous version of that test - and to repeat, don't try this yourself!

Very nice and informative video.

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What is the difference between a MEL, MMEL, KOEL and Equipment List?
When we are determining what equipment must be operational for an aircraft to be considered airworthy we are presented with different kinds of equipment that could be installed on the airplane.
What is the difference between these different types of equipment lists?

Minimum Equipment List (MEL)
Master Minimum Equipment List (MMEL)
Kinds of Operation Equipment List (KOEL)
Equipment list

I would like to have answers for all the different aviation authorities.  US, EASA, Canadian, etc.

Definition of Master Minimum Equipment List from ICAO Master Minimum Equipment List/ Minimum Equipment List Policy and Procedures Manual:

“Master Minimum Equipment List” means a document approved by the Director General that establishes the aircraft equipment allowed to be inoperative  under conditions specified therein for a specific type of aircraft.
A Master Minimum Equipment List (MMEL) is an approved document created specifically to regulate the dispatch of an aircraft type with inoperative equipment. It establishes the aircraft equipment allowed to be inoperative under certain conditions for a specific type of aircraft and still provide an acceptable level of safety. The MMEL contains the conditions, limitations and procedures required for operating the aircraft with these items inoperative. The MMEL forms the basis for development and review of an individual operator's  Minimum equipment List (MEL).

Definition of Minimum Equipment List from the same document:

“Minimum Equipment List” means a document approved by the Director  General that authorizes an operator to dispatch an aircraft with aircraft  equipment inoperative under the conditions specified therein.

and

The MEL is a joint operations and maintenance document prepared for or by  an operator to:
a) identify  the minimum equipment and conditions for an aircraft to maintain the Certificate of Airworthiness in force and to meet the operating rules for the type of operation;
b) define operational procedures necessary to maintain the required level of safety and to deal with inoperative equipment; and
c) define maintenance procedures necessary to maintain the required level of safety and procedures necessary to secure any inoperative equipment.

Regulatory authorities use similar definitions. For example, Transport Canada uses nearly the same definition, replacing 'Director General' with 'minister'. So the ICAO definition can be used as the base.
Kinds of Operation Equipment List (KOEL) are lists one finds in some aircraft POH that indicates what equipment (generally optional) si required for flight under certain conditions. From FAA Advisory Circular Minimum Equipment Requirements for General Aviation Operations under FAR PART 91 AC NO: 91-67:

Kinds of Operations List (KOL). The KOL specifies the kinds of operations (e.g., visual flight rules (VFR), instrument flight rules (IFR), day, or night) in which the aircraft can be operated. The KOL also indicates the installed equipment that may affect any operating limitation. Although the certification rules require this information, there is no standard format; consequently, the manufacturer may furnish it in various ways.

Nice answer.  What is an equipment list though?

An equipment list is a list of every electrical component on the airplane. It gives part numbers, location and effectivity. http://imgur.com/a/2Gysy


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Is there a good reason why modern airliners can't have conventional landing gears (vs. tricycle landing gears)?
By "good reason", I mean a reason that we definitely cannot overcome with reasonable changes in aircraft design, control laws or procedures.
As possible benefits of this I see a much smaller (= lighter) tail landing-gear than the nose landing-gear. Also, an opportunity to reduce tail-strike margins (=increase incidence, reduce landing speed). And of course there is all that space at the front of the fuselage that is freed for other uses (systems, cargo, stairs,...?)
One drawback is of course passenger comfort. But passengers already experience high pitch angles during climb, and we could imagine to have the tail of the aircraft raised up (on a stand?) for boarding/deboarding. Also the APU has be moved (but I don't think this is impossible).

see: http://aviation.stackexchange.com/questions/8550 http://aviation.stackexchange.com/questions/7519

I suspect you would find out one reason when trying to keep straight whilst braking hard on a rain-soaked runway.

@MartinJames, How about differential braking (+ the rudder) ? This is possible with electronic control logics on modern airliners.

Neither answer mentions groundloops or flips; are these not a concern (in a large airliner)?

I would also make the comment that it seems to me (as an enthusiast only) that conventional landing gears would refer to tricycle, since the vast majority of aircraft (at least commercial) are that style vs tail dragger.

How could the wheel at the tail be any smaller when it has to carry the same weight?  I don't see that any of the advantages you've suggested are actual advantages.

@JamesRyan, You are right, the wheels wouldn't be much smaller. But the leg would be considerably smaller, as the wheel can be very close to the fuselage (arguably partially inside the fuselage)

@CGCampbell:  Conventional landing gear came first.  When tricycle landing gear was first introduced, its acceptance was slower than it might have been, because tail wheel aircraft were considered superior for rough, unpaved runways - which describes many more airports then than today.  What you suggest makes some sense except by trying to rename the configuration you are mucking around with tradition.  Pilots learn early what 'conventional' and 'tricycle' means with regard to landing gear.  Since this nomenclature is clear for pilots, what problem exactly are you trying to solve?

@JamesRyan: A tailwheel (for most aircraft layouts) doesn't carry as much weight as a nosewheel does, since it's further from the aircraft's center of mass.

Calling the landing gear on a tail dragger "conventional" sounds like calling the transmission of a stick shift car "standard".

Short answer: Yes. Not one, but several. A taildragger configuration for modern airliners comes with several disadvantages:

Visibility during taxiing is much worse.

Visibility while taxiing is a safety-relevant issue (Picture source)

Braking hard will result in a headstand. And with today's landing speeds, braking hard is needed if you want to fly from standard length runways. Carrier-borne aircraft were the first to feel this consequence of increasing landing speeds, so they converted first to tricycle gears.

That is what is colloquially called a "headstand" (Picture source)

Loading and unloading will become more complicated. The current way of sliding containers manually on a flat deck would need to be replaced by internal winches pulling the containers up the ramp to the door.
Drag during ground roll is much higher as long as tail lift is not sufficient to get the tail up. Take-off distances would also become longer.

In short, it comes down to minimizing field lengths and increasing safety.

Who says there are no taildragger airliners? (Picture source)

breaking hard is relevant if you brake the wheels, but using the reactors, much less, as they are closer to the center of mass along the vertical axis

I up-voted, but I do have a few remarks: visibility: Fair point, but the risk can be considerably reduced with cameras and screens in the cockpit. You could also imagine google-carish sensors to signal obstacles. (even on current a/c configurations) loading/unloading: As suggested in my post, you could imagine a simple ramp at the gate to raise up the tail gear and level the plane. Not a huge inconvenience. drag: Being proportional to v², the increase of drag at the beginning of the run is not big. And you save some drag & friction thanks to no nose gear.

@EricLeibenguth: All your points are valid. My preference is, however, for the most simple solution: Eyeball Mk I and glass. Cameras can get smudged, cables can break, connectors can corrode, electricity can fail. Also, the jacking-up at the ramp needs either ground equipment, or needs to be carried with the aircraft, reducing the weight advantage of a taildragger gear. The less external devices you need, the better. This is less of an issue in the first world, but if you operate in Africa or SE Asia, it certainly is. Drag: The taildragger has a net disadvantage that cannot be discussed away.

@njzk2 That's not true.  Deceleration is deceleration - momentum transfer is forward and the plane will still tip.

@J...: no. It will tip because of the torque of the braking, which exists only if the force is applied away from the center of mass. If the braking has no torque, it won't tip.

These are some really good reasons to favor tricycle gear over taildraggers! +1

@njzk2 Sorry, was thinking backwards... it would, however, be likely to tip during takeoff with forward thrust making torque in the other direction.  You would have to be careful to not accelerate too hard until you had enough windspeed to control your pitch.

One additional reason: Ground steering. On tricycle gear, the pilot has direct steering control over the nose wheel, while on tail-draggers this is (normally?) accomplished by differential braking.

@njzk2: By "reactors" you mean thrust reversers?  Two problems there.  First, you're burning extra fuel to slow down.  Second, what if one or more of the thrust reverser are inoperative?

@jamesqf: I guess that's what I mean, yes. because of the torque inflicted by braking with the wheels, I would have assumed that all taildraggers had to brake using their engine, or renounce braking at all.

@njzk2: No, you can use wheel brakes with a taildragger, just not as hard as with a tricycle gear.

While I agree that visibility would be decreased by a taildragger vs tricycle configuration, I don't think it would be as extreme as in the photo you include. Modern aircraft tend to have much pointier noses than the plane in the photo, with the cockpit farther forwards.

@DavidRicherby: You're right, but I couldn't resist. But still: The downward angle straight ahead is -20°, and pitching the fuselage up like the MD-11 above reduces this to -8°. Given that the pilot now sits much higher, he cannot see the ground for the next 80 - 100 m ahead of him (compared to maybe 20 m with a tricycle gear). This is still significant.

Here's more info on that tipped back Gemini Cargo plane, including more pictures.

@J...:  " it would, however, be likely to tip during takeoff with forward thrust making torque in the other direction."  This is not correct.  On takeoff the wheels are rolling free and are not creating any torque at all.  The thrust of the engines is pretty much in line with the center of mass of the aircraft so there is an insignificant amount of torque.

@SkipMiller: Your're right, but I cannot resist mentioning the Blériot 67, 73, 74 and 75. They all had two engines on the lower wing and two on the upper. During take-off, the pilot could only accelerate with the lower two. Revving up the upper two would result in a headstand, and only when speed was high enough for the tail to control pitch, the two top engines could start to contribute. Nevertheless, Blériot found this concept so convincing that it was kept all the way to the Blériot 155. Those were the times ...

The first picture makes me think, "They don't make 'em like they used to..."

The Gemini picture shows the point I wanted to make.  The wins on modern jet liners are pretty far back on the fuselage.  It's nothing like the first picture where the only thing in front of the wings was the single, huge engine.  Are there any planes with wing-mounted, ducted (non regular-prop) engines that have a rear landing gear?

@Johnny - your airliners.net link has died. Care to stop by and see if you can find the particular post you were referring to and update your comment?

@FreeMan - I can't update a comment, and it's not appropriate to make it an answer here. Here's the Google Search I used to find the info in the first place.

well @Johnny... you can't update a comment, but you can add a new one. Here's the new link for the Gemini Cargo craft doing a wheelie.

I don't think there is a reason they can't have conventional landing gear (tail wheel). But there are simply many advantages to tricycle landing gear:

Aircraft with tricycle landing gear is directionally stable and therefore easier to control especially with cross-wind.
Aircraft with tricycle landing gear has level attitude on the ground, which provides:

Good visibility during taxiing and take-off and landing runs. Many tail-draggers have no forward visibility during taxiing!
Easier loading and unloading.

Since the main gear is aft of centre of gravity, aircraft with tricycle landing gear are less prone to bouncing on touch-down.

Conventional gear has less drag when it is non-retractable, but airliners all have retractable landing gear. The only other advantage is lower weight (the tail-wheel is lighter than nose-wheel), but that difference is not worth all the disadvantages for airliners.

Other answers have given all the good reasons you need, but there is one more that should not be discounted:
In a plane with conventional landing gear, when on the ground, the fuselage is standing at a angle to the ground that is not insignificant.
This means that passengers will have to walk uphill when boarding from the rear door or downhill when boarding from the front door. Same issue when disembarking, of course.
You can try this if you get the chance to visit a DC-3, of which quite a few are still flying with passengers (although the large majority are effectively run as museum pieces, many are still airworthy). A (hypothetical) modern airliner would probably be longer than the DC-3, and the effect hence presumably less pronounced, but still very real.
The effect of the difficulties of such a sloping aisle would be longer turn-around times (and short turn-arounds are the holy grail of low-cost operators), which would have to be offset by equal savings generated by the conventional landing gear.

Not just longer turnaround times, but also safety issues. If a passenger slips and falls you would get a cascade effect that could injure several other pax. Rollaboards, especially spinners with their four free-spinning casters, would compound the effect.

@JamieHanrahan, the "cascade effect" I thought of was the lawsuits filed by passengers, especially in the US. Also, is Rollaboards a rolling suitcase?

"Rollaboard" refers to a rolling bag that's small enough to be a carry-on.

@RickBrant Ah it's not rolla-board, it's roll-aboard!

"rollaboard" has 107,000 Google hits, "roll-aboard" has 38,000. I agree that "roll-aboard" makes more sense. (I don't believe anybody wrote "rolla-board".)

No-one has mentioned jet blast so far.
When a jet liner is starting up, taxiing and accelerating along the runway prior to rotation, its exhaust gases are directed to the rear.
If they were angled downwards, as would be the case with a tail-wheel configuration, the jet blast would rip up the runway and be deflected back at the aircraft itself. This could cause untold damage.

Welcome to aviation.SE. I can agree on everything else, but on the would rip up the runway I strongly object. Damage, maybe, but "rip up" seems a tad excessive.

Jetblast from C-5 engines has very literally "ripped up" tarmac in a few unfortunate instances, so with the right (wrong) combination of factors, I could see things getting beyond "damage" and fully into "ripping up."

Obligatory link.

I think this is a very good point. Not only for the runway's sake, but also for performance: Prior to rotation, a whole component of the thrust vector is perpendicular to the trajectory, and is therefore plainly wasted.

@DavidRicherby - your links to airliners.net all seem to have died. Most likely there's been a reorg there. Care to stop by and fix 'em up?

@FreeMan Durnit. Thanks for letting me know. For this one, the new URL is NL B734 Damaged During Engine Run Up (photos) (with title so people can find it next time the URL changes).

@EricLeibenguth Speaking of engines, thanks to the nose wheel steering, jets can commonly taxi on one engine only, which has a fuel benefit, especially on busy airports will longish takeoff queues.

Once, all airliners were built as taildraggers. The most important reasons none are conventionally geared these days is simple. A taildragger has its center of gravity located behind the main wheels. Therefore, it is insanely easy to groundloop it. This is because taildraggers are not inherently directionally stable. This was not usually a problem when the pilot was ALWAYS expecting the tail to wander to either side on takeoff or landing roll. If a pilot got distracted or lazy and the tail wandered a bit too far  and he was late correcting with rudder, then the plane would swing around in a wild unstoppable loop and often dig in a wingtip, which was not only embarrassing but often damaged the plane. The tricycle gear stopped all that. The new nosegear planes went where the pilot pointed them and stayed there pretty much automatically. The center of gravity was now in front of the main gear and so the groundloop became a thing of the past. Conventional gear is lighter, and allows shorter takeoffs since the plane naturally sits tail low by design, but the instability issue means that its just a no brainer that large powerful airliners will never again
be designed with tailwheels...ever.

All airliners were once taildraggers sounds like you're saying that, for example, the A380 was once a taildragger. Also, your statement on directional stability, while true for landings, is false for takeoffs.

One of the reasons tricycle gears are popular are the fact that they help give plenty of propeller clearance. However, turbofans are by design normally smaller in diameter (They will accelerate a smaller volume of air to a higher velocity in comparison). Hence this is less of a concern. This would also imply that jet engines aligned with the aircraft body would be blasting the runway quite powerfully at full thrust, which would accelerate wear.

How the plane is trimmed is independent of the type of the gear. It depends on how far the centre of lift is from the centre of gravity. Tail-draggers have wings further forward, since their main gear is ahead of CoG, but the trim can remain the same and the longer tail gives the elevator more leverage making it more efficient.

@JanHudec Indeed, I was overthinking this; you are correct.

It is perhaps worth looking at where the main (load-bearing) gear is located on today's airliners - bearing directly on the wing root, which is the structurally strongest part of the aircraft - and where it would be located on a taildragger conversion, considerably further forward.
Taildragger aircraft are universally built with a very forward CG, with both the wing and engines being moved forward to suit.  This happened to be practical with piston-driven propellers, which mainly needed forward-facing mountings and were very heavy relative to the rest of the aircraft.  The main gear was mounted forward of the CG, and often bearing directly on a wing spar which in turn supported the engines.
Imagine doing the same with an MD-80 pattern airliner, with both the engines and wing root towards the rear of the fuselage.  It's then easy to see why tricycle gear is a natural fit for a modern design.

Every answer has some key points as to why airliners today SHOULDN'T have conventional landing gear... but it could be done despite the angle of the aircraft while on the ground (for loading/unloading cargo/passengers) or the drag, or the forward visibility while taxiing, or prop clearance (which wouldn't matter if we're talking about modern jet airliner designs)...
Putting aside safety concerns (prop clearance, stability, etc.) Comfort, loading and unloading concerns, practicality, expenses... Only focusing on why a modern airliner can't have a conventional landing gear... there is no physical reason, to my knowledge, as to why a modern airliner can't be configured as a tail-dragger.
The biggest reason why a modern airliner SHOULDN'T be configured this way... (which is a completely different question than why it CAN'T) would have to be the fact that the jet engines would need to be inline with the fuselage which means that until the tail of the aircraft generates enough lift to raise the tail (making the aircraft level with the ground) the jet engines thrust would be directed towards the ground and with jet engines becoming more and more powerful, this could cause a lot of damage to airport infrastructure and the aircraft itself and would put ground crews in an even more "danger close" proximity to the jet blast...
I'd also venture to say that it may become pretty unstable before it could reach rotation speeds (V1), and I have to wonder how the jet engines would effect the rest of the aircraft when they are spooled up to full power while the tail is still on the ground... I can imagine the engines wanting to stress the engine mounts to the point of some sort of failure whether the engines detach completely or twist the frame of the wings and/or fuselage, maybe even make the aircraft jump off of the ground or even flipping it over and in any case... causing the aircraft to become incapable of sustaining stable flight.
These factors, and more, make a conventional gear configuration completely invalid for a modern airliner. It could be done with enough capital for research and development that would have to change every preexisting design concept to date... reinventing the modern airliner to be something entirely different all together if it were to ever see flight while maintaining the standards and requirements (safety, performance, efficiency, production and operating costs... etc.) that our existing airliners must meet before being put on the market while being cost effective.
Thanks for reading!

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Why did this plane fly in a zigzag pattern?
I was tracking flight NKS739 from LAX to SEA this morning and saw that it made some some odd turns. If you look at the track from FlightRadar24.com you can see that it was heading toward SEA then it took a large turn to the right then to the left then returned to its original course.

This does not look like the usual technical glitch from Flightradar24, and FlightAware shows the same track. There was no weather anywhere in the area. There were many aircraft in the area at the time and none of them made any unusual turns. There are no MOA's in the area (most of it is over a national forest) and none of the other aircraft seem to be avoiding anything in the area. I tried to see if LiveATC.net had a recording but that's Seattle center's area and they don't appear to have a feed for ZSE.
I can't come up with any reason for such an odd deviation. The aircraft arrived in SEA and there didn't seem to be any problem. It immediately turned around and made another flight which was uneventful.
Is there anything that might explain this deviation? Is it just a glitch?

Thunderstorm???

@user3344003 There was no weather anywhere around.

This comment is unavailable for legal reasons.

It is a typical dogleg used to increase spacing between aircraft or delay their arrival. If the airport does not have enough capacity to handle incoming traffic, the air traffic controllers have basically two options to delay arriving aircraft:

reduce their speed
increase their path length

Speed reduction works, but only when the distance to travel is still long. And even then it can only absorb a few minutes as the minimum speed of aircraft is still quite high. At very low speeds the fuel consumption increases drastically, making slow flight unattractive.
Increase of path length is more versatile, it allows for much more time to be added to the flight (fuel is the limiting factor). Basically two types of path lengthening are used: holding patterns and dog legs. In a holding pattern, ATC can stack multiple aircraft on top of each other. A full circuit in a holding pattern takes about 5 minutes so it is a way of absorbing much delay.
When not so much delay is needed, the dog-leg is preferred. By vectoring the aircraft 30 - 60 degrees off-course and then back to the original track, the controller can accurately control the delay and spacing between aircraft. This is used in the initial approach phase and when vectoring to the final approach. Often it is used in combination with speed control; slow flight reduces the size of the dog-leg. However, airlines prefer to fly the optimum endurance speed during such a phase to minimize fuel consumption.
In case of the flight you were tracking, the aircraft was vectored right of its track, then back to cross to the left of its original track and finally back onto the original track. The added distance was approximately 65 nautical miles, which would be about 11 minutes extra flying time.

DeltaLima hits the mark. I used to work there. Undoubtedly traffic management delays going on for some reason, either too much traffic or reduced runway capacity. TBFM issued directly to the aircraft is still in the early stages. I believe most delays are managed by the controllers vectoring, holding or issuing speeds. BTW- speeds are only effective for absorbing very small delays because it takes too much time to work. Old ATC rule; use vectors to get your spacing, speed to keep it.
The "dog leg" vectoring going on looks like it's occurring in the second tier sector, sector 46 (two sectors away from the approach control boundary). The first tier sector may have been at capacity forcing the delays to be pushed out to the high altitude sectors.
FYI - we used to figure about 5 minutes for one turn in hold. Holding needs to be handled at lower altitudes just to keep the plane in the box. I saw someone issue a hold to a 747 at FL390 once and it took the better part of the state to complete one turn!

Hello Iain, welcome to Aviation.SE! Are you a former Air Traffic Controller or still active? I've updated my answer to reflect your rule of thumb for 5 minutes per turn in the hold.

Welcome to the site! Hope you stop in often. Always nice to hear from an ATC. We've got lots of pilots but few controllers and we get a lot of ATC related questions.

Former controller/traffic management and supervisor at ZSE. Retired 2010. The mentions of ERAM give me chills...4 years working to get that beast presentable was hell on the LM engineers and the controllers trying to make it work.

Oh and... Thanks! Good questions here from serious fans of aviation. Good site!


text:
Do helicopters try to take off and land into the wind?
When helicopters take off and land is it preferable to point the nose into the wind? Since a helicopter is providing its own lift does it matter which direction wind is coming from?

Tom, the question about wind and hovering is complex and should be asked separately. It's paragraphs of answer on it's own.  What the wind is doing when hovering is a critical factor and has caused many accidents.

@Simon The first part has been answered pretty well. I'll separate out the hovering part as its own question if I can word it so it isn't too broad. You say it has paragraphs of answer.

In general, helicopters require less power while flying forward (or backwards) compared to hover as translational lift (of Effective Translational Lift, ETL) is produced. Even though the blades are providing lift by rotation, the airspeed experienced by the blades is different if wind is present. For example, if there is headwind $v$, the advancing blades will have an airspeed of $r \omega + v$, while the retreating blade will have an airspeed of $r \omega - v$. As lift is proportional to square of airspeed, the implications are obvious.
For this reason, it is preferable to take off into the wind, rather than take off vertically.

Note that the power required is same for headwind or tailwind. However, tailwind landing (or takeoff) has some disadvantages from stability and power management point of view that is it generally discouraged, like:

In case of landing in tail wind, it is very difficult to abort landing and re-gain forward airspeed. Moving from negative speed to forward speed initially decreases the air speed the main rotor experiences, and the Power Curve shows an initial increase in Power Required. As the helicopter accelerates from negative (ground) speed, the main rotor system loses translational lift before it goes through zero airspeed. As a result, the power required increases, which requires increased pedal application and tail rotor power, precisely when the power requirement is high.
If there is a power failure, the chances of safe landing is reduced.
There is more risk of running out of directional control. In case there is a tailwind, the helicopter vertical tail (and fuselage) may align try to align with the wind, resulting in uncommanded yaw. If not corrected with proper pedal input, this may lead to loss of control.
In case of landing, the possibility of brownout or whiteout conditions is more in case of tailwind.

As I said elsewhere, ETL can be produced by forward speed, but it only requires airflow over the rotor disc from any direction. Also the end, "Note..." with the four bullet points are completely wrong.

How did this get 5 upvotes?

Moving forward through the air helps to make the helicopter more efficient, and it does not matter if that movement comes from hovering in a headwind or from flying forward. For the same reason, a helicopter will climb faster when flying in a spiral than when going straight up.
Technically, flying backwards or sideways will also improve the efficiency of the rotor, but in reverse the helicopter will become directionally unstable. Moving the fuselage sideways will create much more drag, so flying forward is the better choice.

I've been referenced! (Sort of...)

Translational lift comes from airflow over the rotor, not just from forward speed, although that is one way to accomplish it. But translational lift can happen in a hover with just 15knots of wind, and it can come from any compass direction. Although helis are subject to weathervaning, their ability to yaw far outstrips any airplane's, and so reverse directional control is merely a minor nuisance that is easily accomplished.

@rbp: With "forward speed" I meant air speed. The next sentence should have made that clear. For flying in reverse, the instability goes up with speed and the time to double goes down. Slow backwards flight is manageable, but going faster is challenging.

well i know you know the difference, and i think you could write a clearer answer. regarding instability, its unclear where the instability comes from.

Here is something off an FAA document about helicopter performance. https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/media/hfh_ch07.pdf

Winds
Wind direction and velocity also affect hovering, take
off, and
climb performance. Translational lift occurs any time there is
relative airflow over the rotor disk. This occurs whether the
relative airflow is caused by helicopter movement or by the
wind. As wind speed increases, translational lift increases,
resulting in less power required to hover.
The wind direction is also an important consideration.
Headwinds are the most desirable as they contribute to the
greatest increase in performance. Strong crosswinds and
tailwinds may require the use of more tail rotor thrust to
maintain directional control. This increased tail rotor thrust
absorbs power from the engine, which means there is less
power available to the main rotor for the production of
lift. Some helicopters even have a critical wind azimuth or
maximum safe relative wind chart. Operating the helicopter
beyond these limits could cause loss of tail rotor effectiveness.
Takeoff and climb performance is greatly affected by wind.
When taking off into a headwind, effective trans
lational lift
is achieved earlier, resulting in more lift and a steeper climb
angle. When taking off with a tailwind, more distance is
required to accelerate through transla
tion lift.

although this is a citation rather than an answer in the author's own words, it is the only correct answer.

@rbp I agree. It's the only one that has any real detail specific to why a headwind is preferable. I just hate to click accept on a cut-and-paste answer

that's why i added my own answer (hint, hint)

Effective Translational Lift (ETL) is lift generated by airflow over the rotor, and increases the effectiveness of the airfoil by about 15%.  ETL is generated beginning about 15knots of airflow over the rotor.
Assuming the helicopter is landing with the nose pointed into the wind, a headwind decreases the ground speed at which ETL is generated. So a helicopter landing into a 15 knot wind can maintain ETL all the way into a hover (when groundspeed is zero).
Maintaining ETL as long as possible into the landing is important for 2 different reasons:

it requires less power to fly the approach, increasing the safety factor in the case of a go-around, or in high density altitude or high gross weight landings, the requirement to make a run-on landing
by maintaining ETL, there is no chance of entering ring vortex state, which is nearly unrecoverable close to the ground, especially in steep approaches


text:
What is the maximum acceptable gap between the jet bridge and airplane door?
I noticed on an American Airline flight a gap of about 4 inches between the floor of the bridge and the airplane. Wide enough that if the passenger is not paying attention to the floor the foot may slip in between or at  least get stuck.  The flight attendant said there's always a gap. I've seen a gap but not that wide. Is there a maximum acceptable gap?

Could you add a picture to the question.

@Ethan, that doesn't really seem necessary here?

I think this is a good question. Often I feel the danger too, as the section at the edge of the bridge is made of rubber. And the gap worries me about the children boarding the aircraft.

Sometimes the gap varies...

In general, there is a gap between the jet bridge (also called Passenger Boarding Bridges or PBB) and the aircraft. This is due to various reasons like,

To prevent damage to the fuselage
The jet bridge may not fit all the aircraft similarly as their fuselage dimensions are different.

According to FAA Advisory Circular 150/5220-21C,

The PBB should be equipped with sensors to safely prevent
  dangerous contact with the aircraft

The jet bridges at most airports are designed for larger passenger aircraft that are higher off the ground. When a small aircraft utilizes these jet bridges they must use a "ramp" to bridge the gap between the jet bridge and the aircraft.
According to the same Circular,

The maximum horizontal gap allowable along the path to the aircraft can be no
  greater than 1.25 in (32 mm)

So, I guess that a 4 inch gap is not allowed.

I always find it ironic that they have a sensor to protect the aircraft from contact damage but none to prevent the gap from getting too wide and "damaging" a passenger.  Well, I guess aircraft cannot watch out for themselves.

@curious_cat At every flight I took, there was a cabin crew member just behind the door (in the plane), watching carefully the boarding people and greeting them. I assume they would be ready to offer a helping hand to anybody in need.


text:
Drones photography and ground control regulation
I have been having a conversation at the office about whether to use ground control when using drones/UAVs to capture aerial images. The ground control that I am referring too is any permanent survey markers, or Real Time kinematic systems that would aid in improving the spatial accuracy of the data captured. I would imagine that the level of ground control you would use, would depend on the accuracy that you were trying to achieve. I am not sure if this is an aviation question or more GIS related due to using surveying techniques.
Would anyone know if there are any fixed regulation for the use of ground control within Australia?

Welcome to the site. Could you explain what you mean by ground control, and tell us which country is to be considered, as regulation may vary from a country to another.

@Steve V. All good, I full understand regarding the vote.

I have read this several times now and the more I read, the more I am convinced that this is not really aviation-related.

@SentryRaven Yes, it's debatable, although the link about kinematic systems seems to describe something very like WAAS

Thinking about what I wrote, I believe that the question is more geared towards GIS and so I'll post on that site. I am happy for this question to be closed.

I agree to the VTC because it has been answered here: http://gis.stackexchange.com/q/152299/45254


text:
Can airliners benefit from vertical take-off and landing?
A vertical take-off and landing (VTOL) aircraft is one that can hover, take off, and land vertically. We have several military fighter jets, which can takeoff and land vertically (like the Harrier Jump Jet).
Would it be possible to design an airliner (for example as big as a A320 carrying over a hundred people and travelling at 0.85 mach) to perform VTOL?

If your plane as big as an A320 is only carrying 100 people, it is doomed. An actual A320 carries 150-190 people, so you're talking about something that's much more expensive to run than a conventional plane but only earls 50-65% of the revenue.

The closest to what you ask was maybe the Do-31 experimental VTOL transport.

Dornier Do-31 in hover. Picture Source
It was developed to fly supplies to VTOL fighter bases. Makes no sense to have a VTOL fighter fleet if you cannot supply them, right? That was what the Do-31 was for, and the technically successful program folded when NATO abandoned the VTOL strategy in the Sixties.
Compared to regional airliners of its time, the table below shows how much the installation of sufficient thrust has cut performance:

I used the Fokker F-28 for comparison, because it is of similar size and age. As you can see, VTOL easily costs half the payload. 36 fully equipped troops may weigh more than 36 passengers, but they will sit on much lighter seats and have much more frugal accommodations. Note that I used the reduced take-off mass for VTOL flight, but have no information if that mass would have included the full payload. The range is valid only for the higher maximum take-off mass of 24.5 tons, for which a conventional take-off was necessary.
The Do-31 could have fitted the Belgian airline SABENA, which at the time (the Fifties and Sixities of the last century) had the most advanced VTOL strategy of all airlines. They operated regional connections with helicopters, but eventually reverted to regular aircraft well before the Do-31 would had been available. Heli-Air Monaco is still in operation after almost 40 years, but this is more a taxi service than an airline.
Considering not only the increased fuel consumption, but also the increased maintenance cost on more and diverse engines (most VTOL airplanes have dedicated lift engines), the operating cost even at the times of cheap fuel were too high to make the convenience worth it.
I think it is possible to develop a VTOL A320, but it would look very different, would cruise at a lower speed and have a much reduced payload and range. Tickets would be horrendously expensive, though.

EDIT: Thanks to @egid for his suggestion to use mass fractions for comparison. Unfortunately, I found detailed mass data of the F-28 only for a later version, but the expanded table shows the point clearly:

The useful load fraction is the result of dividing the difference between take-off mass and operating mass empty by the take-off mass, showing the sum of payload and fuel as a fraction of take-off mass. Adding lift engines, slow-speed controls and a rear loading ramp clearly reduced what could be carried in VTOL flight.

Your comparison is pretty good but the mtow seems drastically different. Is it that surprising that payload is smaller? What about payload as a fraction of mtow?

@egid: The best I have is payload+fuel as a fraction of MTOW, but you're right, this helps.

Nice, definitely still helps.

Another factor not mentioned is the far higher maintenance cost of the VTOL aircraft (more engines and moving parts) and higher fuel cost. Thus the operating cost per passenger/mile is even more out of whack with conventional aircraft than the numbers based purely on number of passengers and range would suggest.

The V-22 Osprey is similar: 32 troops, MTOW 27.4 tons, useful load fraction 12.4/27.4 = 0.4526, range 1600 km, but cruise speed only 446 km/h.

@jwenting: What part of " the increased maintenance cost on more and diverse engines" was so hard to understand? Maybe I should rephrase that section.

A big part of making an aircraft that can take off and land vertically is the thrust to weight ratio has to be greater than 1 so that it can work against the force of gravity. The Harrier Jump Jet can only take off vertically when it isn't loaded to full capacity (in which case STOL is used).
For a commercial airliner to perform VTOL, taking the A320 as an example, it would require more than 765 kN of thrust, which is 3.2 times the thrust it currently has. Such engineering is infeasible with current technology.

Maximum A320 mass is 78 tonnes, so the weight is ~765 kN.
The two CFM56-5B4's produce 240 kN.

Since the duration needed is relatively short you probably could use rocket engines which have better thrust to mass ratio without the mass of fuel burying you. Especially since you'd only need the lift near the ground. Obviously it wouldn't make any sense, though. Using runways is much cheaper and you'd never get permission to fly passengers with such a contraption. Rocket fuels being dangerous almost by necessity and all. Then again the VTOL version of F-35 doesn't make any sense to me either, so what do I know...

@VilleNiemi "Military Appropriations" and "doesn't make any sense" often go hand-in-hand, until you factor in pork and politics.


text:
What are the differences in door types in a commercial airliner?
I was looking on YouTube at crew training videos. One door caught my attention for a Qantas 767. This door was completely electric and slid up and into the fuselage above, instead of out and to the side like most ones I've seen.
My question is how many different door mechanisms are there on commercial airliners and what are the benefits/disadvantages of each?

This question covers plug-type doors.

The only formal classification of door types that I could find is in 14 CFR 25.807, which specifies the emergency exit requirements for transport category aircraft. It defines Type I-IV, Type A-C, ventral and tailcone exits. The doors you usually enter and leave the aircraft through are probably type I or A:

Type I. This type is a floor-level exit with a rectangular opening of
not less than 24 inches wide by 48 inches high, with corner radii not
greater than eight inches.
Type A. This type is a floor-level exit with a rectangular opening of
not less than 42 inches wide by 72 inches high, with corner radii not
greater than seven inches.

These definitions are quite generic, but Transport Canada has a handy list of Aircraft Exit Profiles that gives specific details (often including the type) for many common aircraft. It says this for the 767 main doors:

B-767-200/300ER (Doors 1L/R FWD, 2L/R AFT)
Note: Some models equipped with 2 additional type 1 exits L/R just forward of wing.

H - 1.88 m (6 ft 2 in) W - 1.07 m (3 ft 6 in); Type A
Equivalent force required to move door control handle from closed/locked position to open position (Normal and Emergency): 12 kg
(26 lbs).
Approximate force required to open a door (lift upward) from unlocked to open: 18 kg (40lbs).
"Counterbalance" opening design.
Inward/upward opening doors.
Manually operated except some models equipped with electrical (normal operation) opening/closing option (FWD Entry Door).
Unique arming/disarming mechanism (lever, indicators).

Release button must be depressed and held to move arming lever to "Slide Armed" mode.
Yellow plastic "Emergency Use Only" (bendable) flag moves upward in front of Door Control Handle when slide is in armed mode.
Double channel slides.

Upward rotating Door Control Handle.
"Uplatch" mechanism holds door in open position. Button must be depressed while door is lowered initial few inches, to release.

You can see from that list that the details can vary a lot, even if the basic designs are very similar.
Coming up with pros and cons for every variant would be tough (especially for me, since I know nothing about it!), but obviously some aircraft are larger than others and that will limit how much space is available for the door mechanism. There are also operational considerations: aircraft that frequently operate out of smaller airports without jet bridges may need to have airstairs, which are usually built into the doors.

You've asked about different doors. I'll skip the standard, rectangular pull in/push out door and the L1011-style door you've mentioned.
Some examples:

A) please add attribution for all your images - a few are watermarked, but source for all is considered polite. B) If you would add some description of the door type and differences between what you've posted, that would be helpful as most of what you've posted appear to fall into the general category of "air-stairs".


text:
In what manner does the Cockpit Voice Recorder timestamps?
See here the transcript of ending moments of Aeroflot Flight 593 . The Wikipedia writes :

(N.B.: All timestamps refer to the flight data recorder time, not GMT or local time.)

What do 2265,2266...2300.....2393..2400 mean? Maybe, they don't mean seconds since CVR can record for only 30 minutes i.e. 1800 seconds..

It says FDR time, not CVR time. But other A310 incidents had a 25 hour FDR, so I don't know where the 2400 is coming from.

If those are minutes, then they spoke one sentence per minute. That seems very, very odd. It seems more likely that they are line numbers - i.e. each statement is simply numbered from 1 - whenever the recording ended.

@FreeMan if they are numbering by statement, that doesn't make sense to label it "time", and then skip a bunch of them.

good point, @fooot, I didn't see the missed numbers.

As described, it is the flight data recorder time. The linked video in that Wikipedia article shows it as seconds. I could not find after how much time Flight 593 crashed, but 2400 seconds equal to 40 minutes.
An FDR typically records around 90 parameters, and certainly can and does monitor all of them all the time, even when several parameters change within a single second.
For the purpose of transcript for this accident, the article on Wikipedia (which is far less technical and detailed than an NTSB investigation report) omits all the entries from FDR recordings which are neither relevant to the story, nor interested to the reader. For example, the reader will care less the input from different sensors between 2303 and 2314.

Since the recording time of a CVR (30 minutes - 2 hours) is less than an FDR (25 hours), they are synchronized for an investigation. This is described here (PDF), for the investigation of Flight 1549:

The CVR and FDR data were synchronized to one another by comparing the
  FDR “Key VHF” parameter with radio transmissions as heard on the CVR
  recording. By comparing the CVR elapsed time (time since the beginning
  of the CVR recording) for radio transmissions to the corresponding FDR
  Subframe Reference Number (SRN) for “Key VHF”, a relationship between
  the CVR elapsed time and the FDR SRN time can be developed.


text:
How do I paint an airplane with an electrostatic paint gun?
I have a Cessna 150 which needs a paint job. As I have experience in painting objects with a spray can, and also in stripping airplanes, I am thinking to paint the Cessna myself.
What are the considerations required for this job? I would enjoy the experience and I'm able to get it prepped and ready. What would be a good electrostatic paint machine for this job? And how can I practice before painting the aircraft?

Note, I know the paints are different, I've heard of something called Imron, as well as Epoxy paints that harden and set.  Suggestions on that would be welcome too.

Paint on an airplane needs to be relatively thin, since an A/C has a large surface area and weight is an issue. It also needs to be tough because of the aggression of relative wind, UV light, and even mechanical due to debris thrown up by the engines. All these factors are quite different from what is needed in home painting. I would suggest migrating this question to Aviation SE, where there are people quite knowledgeable on this topic and who will no doubt set you on your path.

I'd check out the EAA for resources. There will probably be a local chapter and tons of folks with experience building metal aircraft. I wager most do not paint themselves.

I am a retired aircraft painter.
For electrostatic painting: ensure that the aircraft is grounded; ensure that you are grounded to the gun; use compatible electrostatic paint, this is a must.
My suggestion is, unless you are greatly concern about overspraying, to use a normal system on small aircraft, such as suction fed or gravity fed, as it will do the same job. Application is the same for all systems.
Hope this helps


text:
How are weight ranges mentioned for GA airplanes in TCDS?
I'm writing software to integrate a weight and balance chart into my website. Most of the Type Certificate Data Sheets TCDS I've read follow the form:
x to y at z lbs. or less
a to b at c lbs.
Straight line variation between points given.
I want to make sure that I test this against more complex scenarios.  Are there general aviation aircraft that have more than 2 "ranges" or are otherwise exceptional that would make for good test cases?  I am only concerned with GA aircraft at this time.

Are there general aviation aircraft that have more than 2 "ranges"

Yes. I happen to fly one (the PA-28 series, TCDS 2A13).
The loading graphs look like this:

Note also that the aircraft is certificated in both the Normal and Utility category, and the weight/CG ranges are different (Utility being a subset of what's permissible for Normal category operations) - so there are two different operating envelopes that must be considered.

Awesome answer!  Thanks for taking the time to include the graphs.  This will make a great test scenario.

@dazedandconfused The Piper TCDS actually has the graphs in it, so it was just a cut-and-paste job. There are probably more complex examples out there, I just happen to know this one because I have to deal with the funny-shaped graph every time I work a W&B calculation. (I can't think of any aircraft which are not "straight-line variation" between the ranges offhand, so I think you can avoid having to do curve-fitting...)

This is actually very similar to most business jets that I have flown as well.


text:
How can one enter an airliner without stairs or a jet bridge?
I see that big planes (B737, A319 etc.) always need a staircase or a boarding tunnel in order for crew or passengers to enter the cabin since the position of the entry door is quite high (meters above the ground). What solutions are there if none of these options are available? (Except, obviously, for aircraft like DC-9's/MD-82 and 727's which had the rear entrance) How could the pilots get in? Is there some sort of manhole under the aircraft that can be opened to get inside with a sliding staircase or similar?
Living in Africa, I have been to a couple of airstrips where these aircraft do land. Obviously all the airports had stairs but since we cope with some strange situations over here, the question came into my mind. What is the alternative should something go wrong and no traditional means be available to get in the plane?

I don't know if there are any manholes, but what kind of airport would have a runway to encompass that kind of plane and not have stairs?

You are right but my question is just out of curiosity. I live in Tanzania, Africa and there are a couple of remote, very basic airstrips in which I have seen 737's land. They did obviously have stairs in the airport, but, being in Africa, there could be cases where these would be broken or been stolen etc!! So that's were my curiosity came out from. What if a situation like this arises? What are the options?

To get out, they have a rubber-boat-like-slide :)
I doubt there will land any B737 etc if there's no stairs available.

@Robuust Yap!! And getting back up?!?!?!   ;-)

It's a fair question, I'm curious myself but I do believe the general design of airliner just has come with the infrastructure. 6,000+ ft runways, trucks with stairs, giant tankers with jet fuel, parking for everyone (planes and cars), terminals, baggage handling the system just works a certain way. It's much different than a small aircraft that designed to be a little bit more independent. Still needs airports though. Sometimes the whole is just greater than the sum of it's parts.

@p1l0t You know why I asked.. just a couple of months ago I was talking with a BA engineer that came to check some damage done to their B767 that used to fly into Dar es Salaam. He was telling me that the plane smashed into the boarding tunnel causing minor damage do to an incompetent marshaller. They then found out that this lady failed all the exams and was put there just because she was related to the airport manager!! So if something important like this happens, I wouldn't be surprised to hear that a staircase is broken or stolen!!!

When the Southwest 737 landed at the wrong airport in Missouri, how did they get everyone off there?

@abelenky look at the slideshow on top, they used stairs, a boeing 737 is tiny compared to the 747 though

A Delta Boeing 747 landed at Midway after a windscreen crack. There were no stairs, so a local carpenter built some. http://milepoint.com/forums/threads/delta-747-makes-emergency-landing-at-midway-atoll.12909/

@Hugh When I first read that, I thought you were talking about this Midway and was confused as to why they wouldn't have just used a jet bridge, let alone why no stairs would have been available.

Very good answers but this image from @Qantas94Heavy comment link is self explanatory!

lol. Air China seems just the airline where pax would probably be forced to enter through the nosegear :P

What are those in the red square marked area

@FabrizioMazzoni Do I look that stupid? I was referred to the req square marked area at the front, near the nose, below the windows.. Looks like some levers

@Firee They're pitot tubes.

@DannyBeckett Thanks.. Could you please elaborate

@Firee Search Google for pitot static. They measure airspeed.

There are no rear entrances these days, but many smaller jets like the Avro-RJ, Bombardier CRJ series or Embraer regional jets have their own air-stair on the main (front left) door. On the smaller planes the door are hinged at the bottom and the stair are part of the inner side of the door, on the larger ones there may be folding stairs stowed beside the door. B737 can have those too, but it's optional, so many operators choose not to have them (and on second-hand aircraft the operator sometimes bother removing them, but rarely adding them back). I've never heard of them on A320 family as it's a lot higher.
You can get always out using the emergency slides and in cockpit they have a rope to climb down through the side window (where it's too high to jump from it) in case the cockpit door can't be used (happens sometimes).
However the only way in is using stairs. It's hard to imagine you wouldn't be able to scramble at least a ladder even at very remote landing strip where the plane has made an emergency landing. Regular landing will obviously not be done to a place without regular stairs (unless the plane has integral airstairs, of course) and if it was emergency landing you need to bring in mechanics and fuel to get the plane out, brining a ladder is not much extra trouble.
Edit: As Kevin noted below, many aircraft have engineering access hatch in or behind nose wheel well. Not an option for boarding passengers, but the crew can use it when they don't have stairs/ladder handy.

Not all EMB-145's have air-stairs.  The ones operated by ExpressJet had the air-stairs removed and replaced with a door like you'd find on the larger jets.

@casey: Large aircraft manufacturers are always willing to customize their products. For all the types that normally have integral airstairs if the operator does not want them, they can get normal door instead.

I was sure I had used built-in stairs on 737s, thought I was going senile.

@NobodySpecial: You might have. Some have it.

To be complete: On some large aircraft One can also get in via ground-based access hatches. For example, many Boeing aircraft have a ground-accessible E&E access hatch, which can be used in conjunction with the E&E access panel to enter the cabin from the ground. Needless to say, that is not a legal option for passenger entry, but it could potentially be used by crew, and is definitely used by maintenance personnel.

With regard to the A320 series, the A319 can also be fitted with an air stair, check out the Antarctic expedition one

At Cairo West, a joint Egyptian-US military airfield, we once used a fork-lift with an empty pallet to get military personnel on and off a 747-200 when the stairs weren't available. It took awhile. That was the method sometimes used to get JFK on Air Force One (when that airplane was a 707 and never in public view, although one picture got out) to avoid him exacerbating his back troubles by walking up the stairs.
Also, unless modified, 747-100s and -200s can be entered and exited from the ground if you're reasonably fit. At age 75 I could still do it, although I might groan a bit. We used that means on occasion, usually when the stairs had been pulled away, and we needed to get someone into the airplane and didn't want the delay of getting the stairs. Given these days of security concerns, I'm not comfortable with providing the details.

Not exactly private knowledge... ;) http://www.pprune.org/questions/73714-climbing-into-747-through-nose-wheel.html

@Qantas94Heavy So I see. Well, old farts like me are entitled to being excessively cautious.  lol

Here is JFK entering Air Force One using a forklift and pallet.

@AdamDavis Technically, that's not a forklift and pallet.  That's a forklift with a man cage, which is far safer.  Forklift + pallet wouldn't even pass OSHA for commercial use, let alone for use by the President of the United States. :)

Some large aircraft (like Air Force One) have been specially modified to include air stairs so that they don't have to rely on equipment on the ground:

The air stairs are quite heavy though, so most airlines have made the business decision to remove them (to save money) and only operate out of airports that have appropriate ground facilities, including stairs.  If something happened and the stairs were not available on the ground, they would simply divert or reposition to an airport with adequate facilities.

Perhaps a similar design was shown in Firefly http://en.wikipedia.org/wiki/Serenity_(Firefly_vessel)

In Canada's northern areas older large planes are used for freight.  The crew just bring along an aluminum ladder.

On a related note, here's an example of a ladder being used after a landing gear collapse.


text:
Why was my Exhaust Gas Temperature considerably lower than usual?
I have been trying to understand the exact details of an EGT gauge, specifically to understand why on a recent flight it was (for a short time) reading much lower than I would have expected.
I would usually reference the EGT gauge during leaning, and from experience of the  aircraft, am aware that peak RPM usually coincides with between 1400-1500 °F. This is a useful cross check, and reference during leaning.
During a flight yesterday the EGT was reading about 1200 °F when leaned just rich of peak. This felt unusual, and I wondered if there might be something amiss. The OAT was around 10°C (50°F) degrees, and I was in a thin layer of cloud at the time.
Some 20-25 minutes later I was clear of cloud and the EGT was reading 1400 °F which is more what I might expect.
Is there any obvious reason for this?

An EGT is fairly useless unless it provides readings for all cylinders. You normally get large variations between cylinders.  Also the objective isn't a specific temperature, it's a temperature above or below the peak temperature, whatever that happens to be.

You've probably just witnessed the reason high performance piston engines (especially those for warplanes in the last years before the jet era) were equipped with water injection.
Adding water to the intake air, either in the form of vapor or fine droplets (like cloud/fog) will lower combustion temperature and can retard the onset of preignition ("knock" or "detonation") which can damage the engine.  For turbo- or supercharged engines with very high manifold pressure, this can make the difference between literally destroying the engine and simply needing an inspection and oil change after a combat flight with prolonged War Emergency Power.
In a light plane, like most modern GA aircraft, you don't have to worry about manifold pressures in the range of 2-3 atmospheres melting your pistons -- but if your engine is ingesting water, you'll still see a drop in combustion temperature, therefore a similar drop in EGT.  This might be seen as a small loss of power, as well, but if the ingested water is liquid droplets its conversion to steam may actually add a small amount of power while carrying away combustion heat.

Modern materials are also better at handling heat than WW2 materials.

Piston GA engines aren't made with "modern" materials.  Aluminum pistons, for instance, won't take as much heat as the cast iron in most WWII radial engines.  Jet engines are a whole different animal.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Is there an analytical description of the NACA 642-015A profile?
While searching for symmetric profiles I stumbled across different definitions (point-clouds) of the NACA 642-015A profile.

http://airfoiltools.com/airfoil/seligdatfile?airfoil=n64015a-il
http://www.airfoildb.com/airfoils/418.dat

The figure below (skewed) shows the difference in percent chord on the secondary axis (green markers).

What causes these differences and is there a analytical description I could use to create a point-cloud with higher resolution.
Another question is:
The NACA Reports I could find via Google all feature the NACA 642-015 (without the "A"). The difference between NACA 642-015A and NACA 642-015 is an increased thickness of the A-version towards the trailing edge.
Who introduced or developed the A-version?

Have you tried Theory Of Wing Sections by Abbott & Von Doenhoff?

sure, but I could not find the 64_2 015A in them, do you have a profile description in yours?

The "A" simply denotes that the last 20% of the contour are straight lines. This was made to simplify the construction of wing and flaps.

This report seems to include a reference to an analytical description of 6-series airfoils

On your second question: found this reference in Synthesis Of Subsonic Airplane Design by E. Torenbeek:

A modification to the standard series is the A-series in which the sharp trailing edge angle is replaced by a larger one, resulting from straight contours which run from 80% chord backwards.

The paragraph contains a reference to "Theoretical and experimental data for a number of NACA 64-series airfoil sections" which turns out to be NACA LM R6J01

Thanks @Koyovis, this helped a lot, really!. BUT (I'll open a new question for this though, I am not able to use the equations to create the airfoils. I'll get there, thanks!

Actually I just realised that the equation you cited does not recreate the thickness-distribution but the chamber-line, which is a straight line for a symmetrical airfoil.


text:
How to draw NACA 6-Series Airfoils?
In relation to the question on the NACA 64-2A015 airfoil I would like to know how to draw this airfoil.

At least these two reports [1,2] by NASA provide the equation for it.

However, I am not able to find the correct parameters to draw it, so that it would match up with the coordinates given in the table above.

Here is the Matlab code I used:
a = 0.4;
b = 1.0; % caution for NON-unity entries change the equation for h
% c = 1; to simplifiy the equation the chord is set to 1

cl = 1;

g = -1/(b-a) * (a^2 * (1/2  * log(a) -1/4) - b^2 * (1/2 * log(b) -1/4)); % g  = -1/(1-a) * (a^2 * (1/2 * log(a) -1/4) + 1/4)
h = 1/(1-a) * (1/2*(1-a)^2 * log(1-a) -1/4*(1-a)^2)+g; % simplified version for b = 1: h =  1/(b-a) * (1/2*(1-a)^2 * log(1-a) - 1/2 * (1-b)^2 * log(1-b) + 1/4*(1-b)^2 - 1/4*(1-a)^2) + g

x = (0:0.001:1);
y  = cl/(2*pi*(a+b)) * ( 1/(b-a) .* (1/2 * (a-x).^2 .* log(abs(a-x)) - 1/2 .* (b-x).^2 .* log(abs(b-x)) + 1/4 .* (b-x).^2 - 1/4 .* (a-x).^2) - x.*log(x) + g - h.*x); % y = cl/(2*pi*(a+1)) * ( 1/(1-a) .* (1/2 * (a-x).^2 .* log(abs(a-x)) - 1/2 .* (1-x).^2 .* log(abs(1-x)) + 1/4 .* (1-x).^2 + 1/4 .* (a-x).^2) - x.*log(x) + g - h.*x);

L6j01_x = [0, 0.005, 0.0075, 0.0125, 0.025, 0.05, 0.075, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1];
L6j01_y = [0, 0.01193, 0.01436, 0.01815, 0.02508, 0.03477, 0.04202, 0.04799, 0.05732, 0.06423, 0.06926, 0.0727, 0.07463, 0.07487, 0.07313, 0.06978, 0.06517, 0.05956, 0.05311, 0.046, 0.03847, 0.03084, 0.02321, 0.01558, 0.00795, 0.00032];

plot(x,y), axis equal, hold on
plot(L6j01_x, L6j01_y,'ro'), hold off

It seems my Leading Edge definition is not right.
Any suggestions on how to adjust the front part of the airfoil-section?

It's been a while, but I vaguely remember hearing during lectures that the leading edge of an airfoil is a circle. So at some point the line from the equation would transform into a circle with radius unknown, perhaps the NASA report server can help you further.

Jup, you're right. If I see it correctly this is also stated in the figure/drawing from the NASA report above. But I was actually talking about the mismatch of my calculated line (blue) with the red circles. Especially the section 1-40% chord.

Oh right, I assumed the starting point needed to be offset to depart from the nose circle instead of from (0,0).

If you're using Matlab, can it construct a polynom through your data points?

From this reference:
$$c_f=K_1\cdot \frac {t}{c} + K_2(\frac {t}{c})^2 +  K_3(\frac {t}{c})^3 + K_4(\frac {t}{c})^4$$
For the 64A series:

$K_1$ = 8.2125018
$K_2$ = 0.7685596
$K_3$ = 1.4922345
$K_4$ = 3.6130133

$c_f$ is the particular scale factor for this profile. And then the reference continues with this head spinning further explanation:

Now, for a specified family and thickness, the thickness distribution may be determined without iteration. From the thickness, the scale factor is computed from the polynomial function shown above. Then, the scale factor is used to multiply the basic values of the psi and epsilon functions for this airfoil family. These scaled psi and epsilon functions are used in mapping the z-plane to the z'-plane shown in Figure 1. The Joukowski function zeta = z' + 1/z' then maps the z'-plane into the zeta-plane and these results are normalized so that the leading edge is at x=0 and the trailing edge is at x=1.

Hope this helps, my head hurts after reading that.

What do you mean that the chart fails to give the leading edge (LE) radius? It plainly states that the leading edge radius is 1.561% of the chord. Let's just call the chord the unit of "1" to make things easier. Anything larger or smaller is simply a ratio, right? Therefore, the LE radius is 1.561
The plot looks to be symmetrical, mirrored across the chord line. Maybe I am reading too much into this. Regardless, I hope this helps. Other airfoils generally give the upper and lower coordinates, that's why I surmise the plot is symmetrical across the chord line.

The 1- and 6-series airfoils were the first which used a target pressure distribution to define the contour of the surface. This was a major progress in airfoil design but makes it impossible to use a simple equation to exactly describe the contour mathematically. The older, 4-digit series simply used the mean of a collection of successful early airfoils.
I have always used a collection of Fortran subroutines to generate the contour, and it uses tabulated data and splines to generate the shape. Here is a link to a similar collection, but in all cases you will only get an approximation. Use XFOIL or similar code to refine the pressure distribution as desired.


text:
How can an inexperienced pilot tell if the weather is good enough for flying?
When I look at weather in the morning of my flight, it looks good. However, the actual determination can be made later during the day closer to my flight time.
Being a student pilot, how can I efficiently make a decision about flight based on weather? Is there a particular way which can help me?
My flight is at 2PM. When I check weather at 8:30AM using this website I get the METAR for my closest airport as shown in these pictures:

There are a few details available. I want to know how can I make an informed decision at 8:30AM about my flight, instead of waiting until around noon time?

No two people will judge every set of weather conditions exactly the same way.  Learning to judge the weather is part of learning to fly, and the best resources you have for learning that part of flying are the same as the best resources you have for learning all other parts.  Call your FBO and ask them, and/or your instructor.  And remember: As a private pilot, even as a student pilot, you are NEVER required to fly.  Even if the instructor wants you to fly, you always have the option to say "Not today."

I second @JohnR.Strohm's comment to call your FBO/Instructor. Also, remember that whether the weather is acceptable is somewhat dependent on what you're going to do. It may be acceptable for staying in the pattern for takeoff and landing practice but not acceptable to go to a practice area some distance away, and it might be acceptable to go to that practice area but not acceptable for a cross country flight.

This is exactly what the Meteorological exam is for, it's a lot to take in but I'd try and prioritise that exam because it tells you exactly what tools and observations you need to make to fly safely. I fell in to the trap of being over reliant on the instructor to make the decision about the weather and my lesson but the reality was that I was training so I could fly by myself so you need to be able to make conscious and definitive decisions. As a side note, the Metar and Taf are usually the last things I'll look at when planning a flight later that day.

I'm a UK PPL of 14 years, I fly out of North Weald which is on the opposite side on London to Farnborough, so I'm well placed to give you some tips here. First, use the Met Office aviation portal for your planning, most of the services a PPL will use are free. Metars are useless for planning as they only say how the weather is right now, TAFs are good for planning, however they only get you so far as they show you the forecast for one location. The Met Office portal gives you access to the low level forecast charts and spot wind charts, these give the big picture for the whole UK and Europe, including fronts, and forecasts in specific zones. These are published 4 times a day and are good for about 18 hours each.
The weather in the south east of England is very hard to predict accurately, it's getting better as they throw more computing power at it but it's still a challenge. When I do my weather planning I look at the low level chart and spot winds, then I look at the TAFs throughout the area. Heathrow and Gatwick both have 24 hour TAFs, and Farnborough is snuggled between them I'd look at them both and assume the worst from each.
Read the TAFs and look at the briefing charts every day you can until you get a feel for it, experience will give you confidence.
Next set limits. In the SE of England there ain't much topography but you don't want to cruise below 800ft in most places, back before I got an instrument rating I typically set a limit of 1000 ft ceiling, anything below and I wouldn't fly except for circuits, and even than that's pushing it. Wind-wise you need to know your crosswind limit for your airplane and yourself. It doesn't matter if your airplane's rated for 20 knots if you can't hack it, so don't put yourself in a bad position.
Lastly, UK TAFs have a funny quirk. You'll often see PROB for probability in a TAF. The only 2 figures you'll ever see in a UK TAF are PROB 30 and PROB 40, the numbers are for percentage likelihood. PROB 30 means it's unlikely to happen, whereas PROB 40 means it probably will happen. In other words a 40% probability does not mean 40%, but about 60-70%, don't ask me why. So if you see PROB 40 consider it likely.

I am a private pilot with just VFR rating. I check many weather sites before I make a decision as to is it ok for me to fly or not. I call 800-WXbrief, which for US is a very good source, too.
I also have developed friendships with many instructors who have been flying to airports of my potential destinations for many years. I ask them about weather patterns on those itineraries and what to generally expect to happen if worse comes to worse.
I also call the tower of airports on my rout and ask them the weather or any noteworthy emerging situation. The flight following service which is available on demand will sometimes assign you a rout to help you stay in VFR till you reach your destination.
All that said, I have had a couple of times to cut short my flight and land to avoid an emerging convective front and call for a ride back home, or get a room in a hotel!

One might consider reading books about aviation weather.  Weather Flying by Robert Buck is timeless, and is good for VFR and IFR flight.  All my primary and instrument students read it.  I cannot remember any student complaining about having assigned reading in that book.
One way is to start by looking at the PROG charts about two days in advance and getting an idea as to what will be driving the WX for my intended flight(s).  Then less than 24 hours before, one can start looking at TAFs.  Before the evolution of phone WX apps, I used to record the TAFs for my route on a file card.  That way the trends were visible.  Hours before the flight, the METARs were recorded.
This way one has access to the trend in the WX.  If the TAFs remain consistent and the METARs are in line with the TAFs, then at least there is a stable forecast. If the METARs are consistently getting worse than the TAFs, then the forecast is less stable and in an undesirable way.
In addition to watching the forecasts and how they are realized in observations for your intended route and nearby stations, there are other metrics.  If one is a student pilot, doing solo pattern work, the instability of the air will influence the gusts and variability in the wind direction. Sailplane students may look at lift indices, such as K or LI, which are measures of air stability.  Most airplane student pilots will look at gusts of surface winds, but often the gust reporting and forecasting only occurs after the gusts become a factor for students.
Looking at the winds aloft forecast will help one appreciate shear and instability at lower levels.  Certainly if there is allot of differences at lower altitudes at or near where one is operating, a slight change against the forecast could yield difficult surface conditions.
Finally, there is a regional factor.  Much of my instruction has been in the vicinity of the great lakes, and lake effect/enhancement can create localized changes, which are often not reflected in forecasts, but can be predicted by regional knowledge.  Your instructor should help out here.  In the US, areas with unique WX will often have seminars and other talks, where the local effects are discussed.  This is more common in coastal, mountainous and similar areas (Great Lakes included).
One's friend is an understanding of the theory, knowledge of the specifics of the reporting products, and the regional tendencies. Integrating those aspects together will reduce your risk, not only of serious WX situations, but also help you better understand your likelihood of a successful flight or a wasted trip to the airport.

The METAR only gives you a small snapshot in time. They are only good for an hour. They are usually refreshed around 55 minutes past the hour. TAFs, on the other hand, cover a 24 to 30 hour period and they are published 6 times a day (0000, 0600, 1200, 1800).
Routine TAFs are valid for 24-hours.
SPECI is an aviation special weather report issued when there is significant deterioration or improvement in airport weather conditions, such as significant changes of surface winds, visibility, cloud base height and occurrence of severe weather.
Now, lets go back to your wx search at 08:30.
Well, looking at a METAR it is already 2.5 hours OLD.
So, you should look again at the METAR @ 12:00 hrs.  But, again you are 2.5  Hours old.  The point being is when you are a PILOT you are ALWAYS looking at the wx.  No one report is going to be good for any extended period of time.  Even when you are flying you can contact Flight Watch (122.2) or listen over a VOR and get wx while you are in-route.  ALWAYS - Get a WX report and a listing of any TFR's. Your instructor should be drilling into you the need to get a wx report and TFR's before every flight.
If you bust a TFR the first thing the FAA does is check when you received a wx report & TFR report.
OK, so you got a current wx report.  You did the flight to your destination airport. Great Flight!  Did you look ahead?  How is the wx going to be for the return flight?  Flying does not start when you turn the key, it starts hours or days before and ends when you tie-down and close your flight plan.


text:
Is it allowed to launch powered paragliders from national parks?
I'm new to the sport and can't find a written answer. I've been told by local PPG pilots that it is illegal to launch from state/federal parks.
I live near Johnson's Beach, Gulf Islands National Seashore and was told by the officer at the gate entrance:

Yeah, go ahead, use the parking area and beach to the west, we have guys that fly out of there regularly.

FAA has the following informational web page for Alaska National Parks:
https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/systemops/fs/alaskan/advisories/parks/
Which has contact information for parks where flight operations are periodically done.  Several of my pilot friends have used park and forest service airstrips with permission, so it is worth asking.  In Alaska, the parks are generally more open to aircraft, than in the lower 48.
There is also an advisory circular, 91-36C.  https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/systemops/fs/alaskan/advisories/parks/media/ac91-36c.pdf
Some of the lower 48 have strips where permission may be granted, plus forest service has some strips (usually gravel) which permission may be granted to use.  Again, contact the park in question, because they often have local rules which are permissive.
State parks vary. Some states prohibit aircraft of any kind, others permit operations, usually situationally.
Paragliders are just one class of airman who are impacted by park policies. sUAS pilots are also impacted.
Regardless of what you are told verbally, I would only consider ops in a Congress Designated Wilderness with written permission. Those areas have a prohibition of mechanized devices, and that includes hang glider and sUAS activities.

No. According to FAA handbook (page 8-6):

Pilots are requested to maintain a minimum altitude of 2,000 feet
  above the surface of the following: National Parks, Monuments,
  Seashores, Lakeshores, Recreation Areas

USPPA's information brochure also mentioned this:

Most state and national parks are off-limits to launch but do allow
  overflight.

See this question and this one for more details on the 2000' thing; it's a 'please play nice' request, not an actual regulation.

@Pondlife Yes, that's a request. I mentioned that to point out that overflying is allowed but national parks don't generally allow to launch.

The general rule in the US is that the FAA regulates where you can fly, but not where you can land or take off. That's up to the landowner, and any local/state/federal land use laws that apply.
36 CFR 2.17 governs aircraft use in national parks, and it says:

§2.17   Aircraft and air delivery.
(a) The following are prohibited:
(1) Operating or using aircraft on lands or waters other than at
  locations designated pursuant to special regulations.

In other words, you can't take off or land without permission. I couldn't find any information source that lists the permissions (if any) for each park, so unless someone else can find one then you'll just have to ask at each one, as you did.
(This question is closely related.)


text:
Why do outboard spoilers deploy more than the inboard spoiler on the A320?
I was reading in the FCOM about the speedbrakes and ground spoilers when I came across the below lines.
The maximum speedbrake deflection in manual flight is:

40° for spoilers 3 and 4
20° for spoiler 2.

The maximum speedbrake deflection with the autopilot engaged is:

25° for spoilers 3 and 4
12.5° for spoiler 2.

Why is there a difference?

@ymb1 I understood the reason for the question you have linked.  In the above answer you stated single slotted flaps are usually thicker. I read about how Airbus uses high lift wing planforms specifically also how the A320 wing was chosen through the paper you linked. If I posted a question could you answer it ? On the A380 also they had to redesign the wings and high lift devices as mentioned but one point kept coming up to increase flap thickeness and I couldn’t understand why?